Opening Characteristic

There is also a process of diode switching on. In the initial stage of switching on, there is a high transient voltage drop. After a certain period of time, the diode can be in a stable state and has a small tube voltage drop. That is to say, the diode shows obvious "inductance effect" in the initial stage of switching on, and can not respond to the change of forward current immediately. During the forward recovery time, the on-going diode has a much larger peak voltage UFP than the steady state. When the forward current rise rate exceeds 50 A/s, there is a high transient voltage drop in some high voltage diodes. This concept is very important in the fast application of buffer circuits.

In addition to the internal mechanism of the device, the inductance effect of the diode is related to the length of the lead and the magnetic material used in the device package. Inductance effect is most sensitive to the change rate of current, so the higher the rise rate of diode current diF/dt, the higher the peak voltage UFP and the longer the forward recovery time.

Turn-off characteristics

All PN junction diodes will store charge in the form of minority carriers when conducting forward current. Minority carrier injection is the mechanism of conductivity modulation, which leads to the decrease of forward voltage drop (VF). In this sense, it is advantageous. However, when a reverse voltage is suddenly applied to the conducting diode, because a large number of minority carriers are stored in the PN junction during conduction, it will take some time to completely withdraw or neutralize these minority carriers by the cut-off time, that is, it will take a period of time to recover the reverse blocking ability. The reverse recovery process is defined as the reverse recovery time (trr). It is noteworthy that the diode is equivalent to a short circuit state before the blocking capability is restored.

Softness factor S is used to describe the rate at which the reverse recovery current disappears from the maximum IRM. Dirr/dt is an important parameter for reverse recovery current. If the dirr/dt is too large, the reverse peak voltage URM will be too high due to the existence of inductance L in the line, and sometimes strong oscillation will occur, which will damage the diode. The influence of dirr/dt on the reverse characteristic can be expressed by the concepts of soft characteristic and hard characteristic. The softening coefficient S can also be expressed as the amplitude of the reverse peak voltage can be predicted by the above formula. Among them, L is the peak voltage applied to the active device when the total inductance URM of the circuit is reversed recovery of the diode. Its value must be less than the voltage rating of the active device, so it is more practical to use di (rec)/dt to express the softness factor. The total time required to deplete charge storage is defined as reverse recovery time trr. As a measure of switching speed, it is a very important parameter when selecting diodes. Trr for general purpose is about 25s. It can be used in rectifiers and circuits with frequencies below 1kHz, but it can be used in choppers and inverters. In the circuit, the fast recovery diode with TRR below 5S must be selected. In some absorption circuits, fast switching on and soft recovery diodes are required.